BC447 Datasheet, PDF (21/36 Pages) ON Semiconductor – High Voltage Transistors

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APPLICATION CIRCUITS
Driving an Analog-to-Digital Converter With the
THS4271
The THS4271 can be used to drive high-performance
analog-to-digital converters. Two example circuits are
presented below.
The first circuit uses a wideband transformer to convert a
single-ended input signal into a differential signal. The
differential signal is then amplified and filtered by two
THS4271 amplifiers. This circuit provides low
intermodulation distortion, suppressed even-order
distortion, 14 dB of voltage gain, a 50-Ω input impedance,
and a single-pole filter at 100 MHz. For applications
without signal content at dc, this method of driving ADCs
can be very useful. Where dc information content is
required, the THS4500 family of fully differential amplifiers
may be applicable.
5V
VCM
50 Ω (1:4 Ω)
Source 1:2
100 Ω
+
THS4271
_
−5 V
249 Ω
24.9 Ω
100 Ω
22 pF
22 pF
249 Ω
ADS5422
14-Bit, 62 Msps
24.9 Ω
VCM
_
THS4271
+
Figure 78. A Linear, Low Noise, High Gain ADC
Preamplifier
The second circuit depicts single-ended ADC drive. While
not recommended for optimum performance using
converters with differential inputs, satisfactory
performance can sometimes be achieved with
single-ended input drive. An example circuit is shown here
for reference.
THS4271
THS4275
SLOS397E − JULY 2002 − REVISED JANUARY 2004
50 Ω
Source
VI
49.9 Ω ^RT
+5 V
+
THS4271
_
RISO ^{0.1 µF}
16.5 Ω
68 pf
−5 V
Rf
249 Ω
Rg 249 Ω
1.82 kΩ
IN
ADS807
12-Bit,
CM 53 Msps
IN
0.1 µF
NOTE: For best performance, high-speed ADCs should be driven
differentially. See the THS4500 family of devices for more
information.
Figure 79. Driving an ADC With a
Single-Ended Input
Using the THS4271 as a DAC Output Buffer
Two example circuits are presented here showing the
THS4271 buffering the output of a digital-to-analog
converter. The first circuit performs a differential to
single-ended conversion with the THS4271 configured as
a difference amplifier. The difference amplifier can double
as the termination mechanism for the DAC outputs as well.
3.3 V 3.3 V
100 Ω
100 Ω
DAC5675
14-Bit,
400 MSps
249 Ω
124 Ω
249 Ω
249 Ω
+5 V
_
THS4271
+
−5 V
249 Ω
49.9 Ω
RF
LO
Figure 80. Differential to Single-Ended
Conversion of a High-Speed DAC Output
For cases where a differential signaling path is desirable,
a pair of THS4271 amplifiers can be used as output
buffers. The circuit depicts differential drive into a mixer’s
IF inputs, coupled with additional signal gain and filtering.
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